Magnetic sensors are used in many applications such as rotational and torque sensors for motors, proximity sensors for anomaly detection and the like. Magnetic sensors can also be used for biomedical imaging if made sensitive enough.
Long-range proximity detectors and biomedical imaging devices require magnetic field sensitivity on the order of 1 picoTesla/√Hz or better. Superconducting quantum interference devices (SQUIDs) exceed this sensitivity, but are very expensive and power-consumptive since they require cryogenic cooling. Other sensors such as fiber optic magnetometers and fluxgate magnetometers are less expensive than SQUIDs, but in theory have a lower sensitivity on the order of 10 pT/√Hz that in practice has not been achieved.
Also, long-range proximity sensors and magnetic imaging devices would find broader applications if they require very little power or operate passively. This would enable remote and portable operation of magnetic sensors, for which there are applications in proximity sensing and monitoring. Optical magnetometers and fluxgate magnetometers require too much power to be operated remotely; and also, in practice have failed to achieve their theoretical sensitivity levels. Coil type sensors are passive, but have limited frequency operational ranges and low sensitivity.
It would be an advance in the art of magnetic sensing technology to provide a highly sensitive magnetic field sensor that is inexpensive, requires very little operating power, and which can operate at low frequencies.
U.S. Pat. No. 5,675,252 to Podney describes a sensitive, passive magnetic field sensor comprising laminated magnetostrictive and piezoelectric layers. When exposed to a varying magnetic field, strain in the magnetostrictive layers is coupled to the piezoelectric layers, thereby creating a voltage that is detected. Podney employs a large number of layers to provide high sensitivity. Podney does not teach any particular direction for the bias magnetic field or magnetization of the device.
U.S. Pat. No. 6,279,406 to Li et al. also teaches a magnetic field sensor comprising a bonded magnetostrictive layer and a piezoelectric layer. Li et al. fails to teach an optimal configuration for a magnetostrictive/piezoelectric magnetic field sensor.